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2013
Wang, LY, Liu QY, Xu LX, Xie SP.  2013.  Response of mode water and Subtropical Countercurrent to greenhouse gas and aerosol forcing in the North Pacific. Journal of Ocean University of China. 12:222-229.   10.1007/s11802-013-2193-x   AbstractWebsite

The response of the North Pacific Subtropical Mode Water and Subtropical Countercurrent (STCC) to changes in greenhouse gas (GHG) and aerosol is investigated based on the 20th-century historical and single-forcing simulations with the Geophysical Fluid Dynamics Laboratory Climate Model version 3 (GFDL CM3). The aerosol effect causes sea surface temperature (SST) to decrease in the mid-latitude North Pacific, especially in the Kuroshio Extension region, during the past five decades (1950-2005), and this cooling effect exceeds the warming effect by the GHG increase. The STCC response to the GHG and aerosol forcing are opposite. In the GHG (aerosol) forcing run, the STCC decelerates (accelerates) due to the decreased (increased) mode waters in the North Pacific, resulting from a weaker (stronger) front in the mixed layer depth and decreased (increased) subduction in the mode water formation region. The aerosol effect on the SST, mode waters and STCC more than offsets the GHG effect. The response of SST in a zonal band around 40A degrees N and the STCC to the combined forcing in the historical simulation is similar to the response to the aerosol forcing.

Kosaka, Y, Xie SP, Lau NC, Vecchi GA.  2013.  Origin of seasonal predictability for summer climate over the Northwestern Pacific. Proceedings of the National Academy of Sciences of the United States of America. 110:7574-7579.   10.1073/pnas.1215582110   AbstractWebsite

Summer climate in the Northwestern Pacific (NWP) displays large year-to-year variability, affecting densely populated Southeast and East Asia by impacting precipitation, temperature, and tropical cyclones. The Pacific-Japan (PJ) teleconnection pattern provides a crucial link of high predictability from the tropics to East Asia. Using coupled climate model experiments, we show that the PJ pattern is the atmospheric manifestation of an air-sea coupled mode spanning the Indo-NWP warm pool. The PJ pattern forces the Indian Ocean (IO) via a westward propagating atmospheric Rossby wave. In response, IO sea surface temperature feeds back and reinforces the PJ pattern via a tropospheric Kelvin wave. Ocean coupling increases both the amplitude and temporal persistence of the PJ pattern. Cross-correlation of ocean-atmospheric anomalies confirms the coupled nature of this PJIO mode. The ocean-atmosphere feedback explains why the last echoes of El Nino-Southern Oscillation are found in the IO-NWP in the form of the PJIO mode. We demonstrate that the PJIO mode is indeed highly predictable; a characteristic that can enable benefits to society.

Liu, JW, Zhang SP, Xie SP.  2013.  Two types of surface wind response to the East China Sea Kuroshio Front. Journal of Climate. 26:8616-8627.   10.1175/jcli-d-12-00092.1   AbstractWebsite

Effects of the sea surface temperature (SST) front along the East China Sea Kuroshio on sea surface winds at different time scales are investigated. In winter and spring, the climatological vector wind is strongest on the SST front while the scalar wind speed reaches a maximum on the warm flank of the front and is collocated with the maximum difference between sea surface temperature and surface air temperature (SST - SAT). The distinction is due to the change in relative importance of two physical processes of SST-wind interaction at different time scales. The SST front-induced sea surface level pressure (SLP) adjustment (SF-SLP) contributes to a strong vector wind above the front on long time scales, consistent with the collocation of baroclinicity in the marine boundary layer and corroborated by the similarity between the thermal wind and observed wind shear between 1000 and 850 hPa. In contrast, the SST modulation of synoptic winds is more evident on the warm flank of the SST front. Large thermal instability of the near-surface layer strengthens temporal synoptic wind perturbations by intensifying vertical mixing, resulting in a scalar wind maximum. The vertical mixing and SF-SLP mechanisms are both at work but manifest more clearly at the synoptic time scale and in the long-term mean, respectively. The cross-frontal variations are 1.5 m s(-1) in both the scalar and vector wind speeds, representing the vertical mixing and SF-SLP effects, respectively. The results illustrate the utility of high-frequency sampling by satellite scatterometers.

Tomita, H, Xie SP, Tokinaga H, Kawai Y.  2013.  Cloud response to the meandering Kuroshio extension front. Journal of Climate. 26:9393-9398.   10.1175/jcli-d-13-00133.1   AbstractWebsite

A unique set of observations on board research vessel (R/V) Mirai in April 2010 captured a striking cloud hole over a cold meander of the Kuroshio Extension (KE) east of Japan as corroborated by atmospheric soundings, ceilometer, shipboard radiation data, and satellite cloud images. Distinct differences were also observed between the warm meander farther to the north and warm water south of the KE. The atmosphere is highly unstable over the warm meander, promoting a well-mixed marine atmospheric boundary layer (MABL) and a layer of solid stratocumulus clouds capped by a strong inversion. Over the warm water south of the KE, MABL deepens and is decoupled from the ocean surface. Scattered cumulus clouds develop as captured by rapid variations in ceilometer-derived cloud base. The results show that the meandering KE front affects the entire MABL and the clouds. Such atmospheric response can potentially intensify the baroclinicity in the lower atmosphere.

Seo, H, Xie SP.  2013.  Impact of ocean warm layer thickness on the intensity of hurricane Katrina in a regional coupled model. Meteorology and Atmospheric Physics. 122:19-32.   10.1007/s00703-013-0275-3   AbstractWebsite

The effect of pre-storm subsurface thermal structure on the intensity of hurricane Katrina (2005) is examined using a regional coupled model. The Estimating Circulation and Climate of Ocean (ECCO) ocean state estimate is used to initialize the ocean component of the coupled model, and the source of deficiencies in the simulation of Katrina intensity is investigated in relation to the initial depth of 26 A degrees C isotherm (D26). The model underestimates the intensity of Katrina partly due to shallow D26 in ECCO. Sensitivity tests with various ECCO initial fields indicate that the correct relationship between intensity and D26 cannot be derived because D26 variability is underestimated in ECCO. A series of idealized experiments is carried out by modifying initial ECCO D26 to match the observed range. A more reasonable relationship between Katrina's intensity and pre-storm D26 emerges: the intensity is much more sensitive to D26 than to sea surface temperature (SST). Ocean mixed layer process plays a critical role in modulating inner-core SSTs when D26 is deep, reducing mixed layer cooling and lowering the center pressure of the Katrina. Our result lends strong support to the notion that accurate initialization of pre-storm subsurface thermal structure in prediction models is critical for a skillful forecast of intensity of Katrina and likely other intense storms.

Park, HS, Xie SP, Son SW.  2013.  Poleward stationary eddy heat transport by the Tibetan Plateau and equatorward shift of westerlies during northern winter. Journal of the Atmospheric Sciences. 70:3288-3301. AbstractWebsite

The orographic effect of the Tibetan Plateau on atmospheric poleward heat transport is investigated using an atmospheric general circulation model. The linear interference between the Tibetan Plateau-induced winds and the eddy temperature field associated with the land-sea thermal contrast is a key factor for enhancing the poleward stationary eddy heat transport. Specifically, Tibetan Plateau-induced stationary waves produce northerlies over the cold eastern Eurasian continent, leading to a poleward heat transport. In another hot spot of stationary eddy heat transport over the eastern North Pacific, Tibetan Plateau-induced stationary waves transport relatively warm marine air northward.In an experiment where the Tibetan Plateau is removed, the poleward heat transport is mostly accomplished by transient eddies, similar to the Southern Hemisphere. In the presence of the Tibetan Plateau, the enhanced stationary eddy heat transport is offset by a comparable reduction in transient eddy heat transport. This compensation between stationary and transient eddy heat transport is seen in observed interannual variability. Both the model and observations indicate that an enhanced poleward heat transport by stationary waves weakens transient eddies by decreasing the meridional temperature gradient and the associated westerlies in midlatitudes.

Xie, SP, Lu B, Xiang BQ.  2013.  Similar spatial patterns of climate responses to aerosol and greenhouse gas changes. Nature Geoscience. 6:828-832.   10.1038/ngeo1931   AbstractWebsite

Spatial variations in ocean warming have been linked to regional changes in tropical cyclones(1), precipitation(2,3) and monsoons(4). But development of reliable regional climate projections for climate change mitigation and adaptation remains challenging(5). The presence of anthropogenic aerosols, which are highly variable in space and time, is thought to induce spatial patterns of climate response that are distinct from those of well-mixed greenhouse gases(4,6-9) Using CMIP5 climate simulations that consider aerosols and greenhouse gases separately, we show that regional responses to changes in greenhouse gases and aerosols are similar over the ocean, as reflected in similar spatial patterns of ocean temperature and precipitation. This similarity suggests that the climate response to radiative changes is relatively insensitive to the spatial distribution of these changes. Although anthropogenic aerosols are largely confined to the Northern Hemisphere, simulations that include aerosol forcing predict decreases in temperature and westerly wind speed that reach the pristine Southern Hemisphere oceans. Over land, the climate response to aerosol forcing is more localized, but larger scale spatial patterns are also evident. We suggest that the climate responses induced by greenhouse gases and aerosols share key ocean-atmosphere feedbacks, leading to a qualitative resemblance in spatial distribution.

Xie, SP.  2013.  Advancing climate dynamics toward reliable regional climate projections. Journal of Ocean University of China. 12:191-200.   10.1007/s11802-013-2277-7   AbstractWebsite

With a scientific consensus reached regarding the anthropogenic effect on global mean temperature, developing reliable regional climate projections has emerged as a new challenge for climate science. A national project was launched in China in 2012 to study ocean's role in regional climate change. This paper starts with a review of recent advances in the study of regional climate response to global warming, followed by a description of the Chinese project including the rationale, objectives, and plan for field observations. The 15 research articles that follow in the special issue are highlighted, representing some of the initial results from the project.

2012
Chang, CH, Xie SP, Schneider N, Qiu B, Small J, Zhuang W, Taguchi B, Sasaki H, Lin XP.  2012.  East Pacific ocean eddies and their relationship to subseasonal variability in Central American wind jets. Journal of Geophysical Research-Oceans. 117   10.1029/2011jc007315   Abstract

Subseasonal variability in sea surface height (SSH) over the East Pacific warm pool off Central America is investigated using satellite observations and an eddy-resolving ocean general circulation model. SSH variability is organized into two southwest-tilted bands on the northwest flank of the Tehuantepec and Papagayo wind jets and collocated with the thermocline troughs. Eddy-like features of wavelength similar to 600 km propagate southwestward along the high-variance bands at a speed of 9-13 cm/s. Wind fluctuations are important for eddy formation in the Gulf of Tehuantepec, with a recurring interval of 40-90 days. When forced by satellite wind observations, the model reproduces the two high-variance bands and the phase propagation of the Tehuantepec eddies. Our observational analysis and model simulation suggest the following evolution of the Tehuantepec eddies. On the subseasonal timescale, in response to the gap wind intensification, a coastal anticyclonic eddy forms on the northwest flank of the wind jet and strengthens as it propagates offshore in the following two to three weeks. An energetics analysis based on the model simulation indicates that besides wind work, barotropic and baroclinic instabilities of the mean flow are important for the eddy growth. Both observational and model results suggest a re-intensification of the anticyclonic eddy in response to the subsequent wind jet event. Off Papagayo, ocean eddy formation is not well correlated with local wind jet variability. In both the Gulfs of Tehuantepec and Papagayo, subseasonal SSH variability is preferentially excited on the northwest flank of the wind jet. Factors for this asymmetry about the wind jet axis as well as the origins of wind jet variability are discussed.

Yang, L, Du Y, Xie SP, Wang DX.  2012.  An interdecadal change of tropical cyclone activity in the South China Sea in the early 1990s. Chinese Journal of Oceanology and Limnology. 30:953-959.   10.1007/s00343-012-1258-9   Abstract

Tropical cyclone (TC) genesis in the South China Sea (SCS) during 1979-2008 underwent a decadal variation around 1993. A total of 55 TCs formed in the SCS from May to September during 1994-2008, about twice that during 1979-1993 (27). During the TC peak season (July-September, JAS), there were 43 TCs from 1994-2008, but only 17 during 1979-1993. For July in particular, 13 TCs formed from 1994-2008, but there were none during 1979-1993. The change in TC number is associated with changes of key environmental conditions in atmosphere and ocean. Compared to 1979-1993, the subtropical high was significantly weaker and was displaced more eastward during 1994-2008. In the former period, a stronger subtropical high induced downward flow, inhibiting TC formation. In the latter period, vertical wind shear and outgoing longwave radiation all weakened. Mid-level (850-500 hPa) humidity, and relative vorticity were higher. Sea surface temperature and upper layer heat content were also higher in the area. All these factors favor TC genesis during the latter period. The decadal change of TC genesis led to more landfalling TCs in Southern China during the period 1994-2008, which contributed to an abrupt increase in regional rainfall.

Tokinaga, H, Xie SP, Deser C, Kosaka Y, Okumura YM.  2012.  Slowdown of the Walker circulation driven by tropical Indo-Pacific warming. Nature. 491:439-+.   10.1038/nature11576   Abstract

Global mean sea surface temperature (SST) has risen steadily over the past century(1,2), but the overall pattern contains extensive and often uncertain spatial variations, with potentially important effects on regional precipitation(3,4). Observations suggest a slowdown of the zonal atmospheric overturning circulation above the tropical Pacific Ocean (the Walker circulation) over the twentieth century(1,5). Although this change has been attributed to a muted hydrological cycle forced by global warming(5,6), the effect of SST warming patterns has not been explored and quantified(1,7,8). Here we perform experiments using an atmospheric model, and find that SST warming patterns are the main cause of the weakened Walker circulation over the past six decades (1950-2009). The SST trend reconstructed from bucket-sampled SST and night-time marine surface air temperature features a reduced zonal gradient in the tropical Indo-Pacific Ocean, a change consistent with subsurface temperature observations(8). Model experiments with this trend pattern robustly simulate the observed changes, including the Walker circulation slowdown and the eastward shift of atmospheric convection from the Indonesian maritime continent to the central tropical Pacific. Our results cannot establish whether the observed changes are due to natural variability or anthropogenic global warming, but they do show that the observed slowdown in the Walker circulation is presumably driven by oceanic rather than atmospheric processes.

Kosaka, Y, Chowdary JS, Xie SP, Min YM, Lee JY.  2012.  Limitations of Seasonal Predictability for Summer Climate over East Asia and the Northwestern Pacific. Journal of Climate. 25:7574-7589.   10.1175/jcli-d-12-00009.-1   Abstract

Predictability of summer climate anomalies over East Asia and the northwestern Pacific is investigated using observations and a multimodel hindcast ensemble initialized on 1 May for the recent 20-30 yr. Summertime East Asia is under the influence of the northwestern Pacific subtropical high (PASH). The Pacific-Japan (PJ) teleconnection pattern, a meridional dipole of sea level pressure variability, affects the northwestern PASH. The forecast models generally capture the association of the PJ pattern with the El Nino-Southern Oscillation (ENSO).|The Silk Road pattern, a wave train along the summer Asian jet, is another dominant teleconnection that influences the northwestern PASH and East Asia. In contrast to the PJ pattern, observational analysis reveals a lack of correlations between the Silk Road pattern and ENSO. Coupled models cannot predict the temporal phase of the Silk Road pattern, despite their ability to reproduce its spatial structure as the leading mode of atmospheric internal variability. Thus, the pattern is rather unpredictable at monthly to seasonal lead, limiting the seasonal predictability for summer in East Asia.|The anomalous summer of 2010 in East Asia is a case in point, illustrating the interference by the Silk Road pattern. Canonical anomalies associated with a decayed El Nino and developing La Nina would have the PJ pattern bring a cold summer to East Asia in 2010. In reality, the Silk Road pattern overwhelmed this tendency, bringing a record-breaking hot summer instead. A dynamical model experiment indicates that European blocking was instrumental in triggering the Silk Road pattern in the 2010 summer.

Li, G, Xie SP.  2012.  Origins of tropical-wide SST biases in CMIP multi-model ensembles. Geophysical Research Letters. 39   10.1029/2012gl053777   AbstractWebsite

Long-standing simulation errors limit the utility of climate models. Overlooked are tropical-wide errors, with sea surface temperature (SST) biasing high or low across all the tropical ocean basins. Our analysis based on Coupled Model Intercomparison Project (CMIP) multi-model ensembles shows that such SST biases can be classified into two types: one with a broad meridional structure and of the same sign across all basins that is highly correlated with the tropical mean; and one with large inter-model variability in the cold tongues of the equatorial Pacific and Atlantic. The first type can be traced back to biases in atmospheric simulations of cloud cover, with cloudy models biasing low in tropical-wide SST. The second type originates from the diversity among models in representing the thermocline depth; models with a deep thermocline feature a warm cold tongue on the equator. Implications for inter-model variability in precipitation climatology and SST threshold for convection are discussed. Citation: Li, G., and S.-P. Xie (2012), Origins of tropical-wide SST biases in CMIP multi-model ensembles, Geophys. Res. Lett., 39, L22703, doi: 10.1029/2012GL053777.

Alexander, MA, Seo H, Xie SP, Scott JD.  2012.  ENSO's Impact on the Gap Wind Regions of the Eastern Tropical Pacific Ocean. Journal of Climate. 25:3549-3565.   10.1175/jcli-d-11-00320.1   Abstract

The recently released NCEP Climate Forecast System Reanalysis (CFSR) is used to examine the response to ENSO in the northeast tropical Pacific Ocean (NETP) during 1979-2009. The normally cool Pacific sea surface temperatures (SSTs) associated with wind jets through the gaps in the Central American mountains at Tehuantepec, Papagayo, and Panama are substantially warmer (colder) than the surrounding ocean during El Nino (La Nina) events. Ocean dynamics generate the ENSO-related SST anomalies in the gap wind regions as the surface fluxes damp the SSTs anomalies, while the Ekman heat transport is generally in quadrature with the anomalies. The ENSO-driven warming is associated with large-scale deepening of the thermocline; with the cold thermocline water at greater depths during El Nino in the NETP, it is less likely to be vertically mixed to the surface, particularly in the gap wind regions where the thermocline is normally very close to the surface. The thermocline deepening is enhanced to the south of the Costa Rica Dome in the Papagayo region, which contributes to the local ENSO-driven SST anomalies. The NETP thermocline changes are due to coastal Kelvin waves that initiate westward-propagating Rossby waves, and possibly ocean eddies, rather than by local Ekman pumping. These findings were confirmed with regional ocean model experiments: only integrations that included interannually varying ocean boundary conditions were able to simulate the thermocline deepening and localized warming in the NETP during El Nino events; the simulation with variable surface fluxes, but boundary conditions that repeated the seasonal cycle, did not.

Richter, I, Xie S-P, Wittenberg AT, Masumoto Y.  2012.  Tropical Atlantic biases and their relation to surface wind stress and terrestrial precipitation. Climate Dynamics. 38:985-1001.   10.1007/s00382-011-1038-9   Abstract
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Chowdary, JS, Xie S-P, Tokinaga H, Okumura YM, Kubota H, Johnson N, Zheng X-T.  2012.  Interdecadal Variations in ENSO Teleconnection to the Indo-Western Pacific for 1870-2007. Journal of Climate. 25:1722-1744.   10.1175/jcli-d-11-00070.1   Abstract
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Tokinaga, H, Xie S-P, Timmermann A, McGregor S, Ogata T, Kubota H, Okumura YM.  2012.  Regional Patterns of Tropical Indo-Pacific Climate Change: Evidence of the Walker Circulation Weakening. Journal of Climate. 25:1689-1710.   10.1175/jcli-d-11-00263.1   Abstract
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Sasaki, H, Xie S-P, Taguchi B, Nonaka M, Hosoda S, Masumoto Y.  2012.  Interannual variations of the Hawaiian Lee Countercurrent induced by potential vorticity variability in the subsurface. Journal of Oceanography. 68:93-111.   10.1007/s10872-011-0074-8   Abstract
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Xie, S-P, Kubokawa A, Kobashi F, Mitsudera H.  2012.  New developments in mode-water research: an introduction. Journal of Oceanography. 68:1-3.   10.1007/s10872-011-0090-8   Abstract
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Oshima, K, Tanimoto Y, Xie SP.  2012.  Regional Patterns of Wintertime SLP Change over the North Pacific and Their Uncertainty in CMIP3 Multi-Model Projections. Journal of the Meteorological Society of Japan. 90A:385-396.   10.2151/jmsj.2012-A23   Abstract

Regional patterns of wintertime sea level pressure (SLP) trends over the North Pacific and their uncertainty were investigated based on the phase 3 of the Coupled Model Intercomparison Project (CMIP3) multi-model projections under the Special Report on Emissions Scenarios (SRES) A1B emission scenario for the 21st century (2000-2099). While the 24-model ensemble mean of the 100-yr SLP trend over the North Pacific shows a northward shift of the Aleutian low (AL), regional patterns of the SLP change vary among the models. Projected changes deepen the AL in several models but it shifts northward in some others. The different response of the AL results in a large inter-model spread over the North Pacific, which is largest of the Northern Hemisphere and comparable in magnitude to the ensemble mean in the same region. This large spread means a high degree of uncertainty in the 100-yr SLP trend over the North Pacific.|For the total uncertainty in the SLP trends over the North Pacific, we examined the relative importance of the internal climate variability and model uncertainty due to different treatments of physical processes and computational scheme. To evaluate each of contributions, a single-realization ensemble using a subset of 10 CMIP3 models is compared to a multi-realization ensemble for the same models in the A1B projections. Additionally the control simulations under preindustrial conditions are examined to evaluate the background internal variability in each of the CMIP3 models. Our analysis shows that both the model uncertainty and internal climate variability contribute to the total uncertainty in the 100-yr SLP trends during the 21st century, while the internal climate variability largely explains the total uncertainty in the 50-yr SLP trends during the first half of the 21st century.|The changes in surface heat flux and North Pacific subtropical gyre in association with the different response of the AL affect regional patterns of the sea surface temperature trends among models.

Xu, L, Xie S-P, Liu Q, Kobashi F.  2012.  Response of the North Pacific subtropical countercurrent and its variability to global warming. Journal of Oceanography. 68:127-137.   10.1007/s10872-011-0031-6   Abstract
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Li, JX, Wang GH, Xie SP, Zhang R, Sun ZY.  2012.  A winter warm pool southwest of Hainan Island due to the orographic wind wake. Journal of Geophysical Research-Oceans. 117   10.1029/2012jc008189   Abstract

A winter warm pool off the southwest coast of Hainan Island is uncovered from high resolution satellite measurements and field observations. The warm pool is characterized by warm temperature relative to the surroundings. It forms in October, intensifies from November to next January, and decays in February. Our results show that the wind wake in the northeast winter monsoon due to the orographic blockage by mountains of Hainan Island plays an important role in generating the warm pool by reducing surface latent heat flux. The core temperature of the warm pool is correlated with the El Nino and Southern Oscillation.

Weller, E, Feng M, Hendon H, Ma J, Xie SP, Caputi N.  2012.  Interannual Variations of Wind Regimes off the Subtropical Western Australia Coast during Austral Winter and Spring. Journal of Climate. 25:5587-5599.   10.1175/jcli-d-11-00324.1   Abstract

Off the Western Australia coast, interannual variations of wind regime during the austral winter and spring are significantly correlated with the Indian Ocean dipole (IOD) and the southern annular mode (SAM) variability. Atmospheric general circulation model experiments forced by an idealized IOD sea surface temperature anomaly field suggest that the IOD-generated deep atmospheric convection anomalies trigger a Rossby wave train in the upper troposphere that propagates into the southern extratropics and induces positive geopotential height anomalies over southern Australia, independent of the SAM. The positive geopotential height anomalies extended from the upper troposphere to the surface, south of the Australian continent, resulting in easterly wind anomalies off the Western Australia coast and a reduction of the high-frequency synoptic storm events that deliver the majority of southwest Australia rainfall during austral winter and spring. In the marine environment, the wind anomalies and reduction of storm events may hamper the western rock lobster recruitment process.